1. Field of the Invention
This invention relates to indicating techniques of vehicle speed, and more particularly in a preferred embodiment to methods and apparatus for evaluating speed signals placed on a serial data communications link and for using such data to provide digitally indicated vehicle speed information on a display.
2. Description of the Prior Art
In prior art digital speedometer systems, such as the ones described in the article entitled "Microprocessor Control for Liquid Crystal Instrument Panel" by O. Shimizu, H. Gotoh, and H. Arai appearing as paper No. 820101 (P-103) of SAE's Electronic Displays and Information Systems and On-Board Electronics (referred to as the Shimuzu arrangement) and in the Article entitled "A Comprehensive Study of Digital Speedometer" by M. Moriyama, T. Kurovama and H. Shinkai appearing as paper No. 811419 of SAE Transaction Volume 90, (referred to as the Moriyama arrangement) require direct access to a speed sensor. This is in contrast to the present system wherein an engine control unit places information on the communications bus at predetermined intervals.
Both the Shimizu and Moriyama arrangements are the conventional type of electronic digital speedometers comprised of basically a timing pulse, means for generating gate, reset and latch signals at predetermined sequential intervals, a counter to count the number of speed pulses generated for a given gate time in proportion to the vehicle speed, a display driver to latch an output from the speed counter until the next succeeding count is completed and an indicator (LCD, vacuum fluorescent display, etc.) to digitally indicate the value latched by the display driver.
In both arrangements, steps had to be taken to alleviate a display flicker problem caused by the inconsistency in the timing of applying the gate signal in synchronization with the timing of counting the speed pulses.
In the Shimizu arrangement, software filtering (a continuous pulse interval averaging method) was employed to satisfy a flickering display as well as a fast response. A computer counts the number of pulses in 0.36 seconds, which is the display update cycle, and at the same time, it measures the time interval from the first pulse to the last pulse that is gated by the computer. From these two data, an average pulse interval is calculated. Then an algorithm is employed to calculate speed. The latest speed before filtering is determined by dividing the latest average pulse interval into a predetermined constant. The latest speed after filtering is calculated by summing the latest speed before filtering multiplied by another constant together with the product of the displayed speed and still another constant. But at very low speeds, the speed pulse in the Shimizu arrangement is so discrete that the software filtering method requires changes to yield appropriate display responses.
In the Moriyama arrangement, hardware is used to alleviate the gate/speed pulse synchronization and low speed problems encountered by the Shimizu arrangement. Such problems cause display flicker.
Both of the above arrangements employed techniques to resolve specific problems resulting from the limitations of the apparatus used to derive speed indications.
Another prior art solution to the flickering of the electronic dashboard display problem is described in an IEEE Transactions on Industrial Electronics, Vol. 1E-30, No. 2, May 1983. There, a software solution to minimize flickering of the display employs an algorithm that prevents changes in the displayed value of .+-.1.0 km/hr if the vehicle is driven at nearly constant speed.
Approximately every half second, the vehicle speed is measured by counting interrupt pulses generated by the speed sensor. The result is compared with the last displayed value of the road speed. If this comparison provides only a small and temporary difference between these two values, then the display is not changed. If there is a positive or negative difference for some measurement cycles, then the displayed speed is incremented or decremented. But if the comparison provides a very great difference e.g., acceleration, then the new vehicle speed is displayed immediately. This solution, a hysteresis approach, seems to be a step towards minimizing flicker but the inhibited changes in the display occurring with small and temporary differences in speed and the immediate update of the display due to acceleration might provide moments of uncertainty regarding the speed of the vehicle the amount of hystresis which controls the amount of flicker also appears to introduce an unavoidable error in the display. Hence the more flicker is reduced the larger the error could be.